<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <meta http-equiv="X-UA-Compatible" content="ie=edge">
  <title>Rainbow Border Animation</title>
  <style>
    body {
      height: 100vh;
      margin: 0;
    }
    .flex-center {
      display: flex;
      justify-content: center;
      align-items: center;
    }
    @property --angle {
      syntax: "<angle>";
      initial-value: 0deg;
      inherits: false;
    }
    .rainbow-border {
      width: 200px;
      height: 200px;
      background-image: conic-gradient(from var(--angle),
      red, orange, yellow, green, cyan, blue, violet, red
      );
      animation: rotate 3s linear infinite;
    }
    .inner {
      height: calc(100% - 20px);
      width: calc(100% - 20px);
      background-color: #333;
      color: white;
    }
    @keyframes rotate {
      0% {
        --angle: 0deg;
      }
      100% {
        --angle: 360deg;
      }
    }
  </style>
</head>
<body class="flex-center">
<div class="rainbow-border flex-center">
  <div class="inner flex-center">SOME</div>
</div>
</body>
</html>